It is proposed that a detailed, quantitative investigation of the photochemistry of rhodopsin, 11-cis retinal, and related structures be made. The excitation energy dependence of rhodopsin bleaching will be studied by irradiating into its second and third transitions. An analogous study of all-trans and 11-cis retinal is proposed. The unusual temperature dependence of the photoisomerization of 11-cis retinal can be understood if there is a dependence of the cis-trans isomerization process upon the geometry of the 12-13 single bond. In order to prove this idea, it is proposed that two synthetic retinals be prepared and their photochemical properties studied. The temperature dependence of the rhodopsin bleaching efficiency will be studied although we do not expect any unusual behavior. In order to understand the nature of the two step photo-isomerization process in rhodopsin, specifically searching for a prelumirhodopsin type intermediate since it is known that this pigment bleaches to all-trans retinal stereospecifically. Variable temperature cryogenic techniques will be employed to study the spectral and photochemical properties of this and subsequent intermediates. It is hoped that this study wil lend to a better understanding of the molecular events that occur following light absorption by the visual pigment rhodopsin.